Human blood vessels contribute to the vascularization of the thymic implant. Confocal microscopy of cryostat sections from the thymic graft of a CB/Thy-NOD/SCID mouse stained by 2-color immunofluorescence with an anti-human–specific CD34 MoAb (a, b, and d, red fluorescence) and an anti–PECAM-1 polyclonal Ab (a, b, and d, green fluorescence) or with isotype-matched control antibodies (c). Colocalization of the 2 markers (yellow) identifies numerous human vascular structures within the thymic parenchyma, as well as in the subcapsular region of the graft (a, arrows). Human vessels are also seen to penetrate the mouse kidney at the interface with the thymic graft (b, interface marked by dots). (d) Composite of 4 consecutive microscopic fields acquired from a section stained as above showing numerous human vessels infiltrating the mouse kidney at the interface with the thymic graft. Bar in a, b, and c = 42 μm; bar in d = 32 μm.